Conventional methods for welding a tube to a tubesheet include gas metal arc welding and gas tungsten are welding. Gas metal arc welding uses a consumable metal wire as one electrode and the parts as another electrode, and moves the consumable metal wire (or the parts) to draw an arc and weld the parts together. Gas tungsten arc welding uses a non-consumable tungsten alloy electrode to draw an arc and a consumable filler metal wire that is fed into the arc to weld parts together. The welding is accompanied by a gas (such as a mixture of argon and carbon dioxide) to prevent oxidation and stabilize the arc. Such gas metal/gas tungsten arc welding is well known. In a conventional gas metal arc welding technique, solid metal wire or metal core wire (i.e., an annular-solid wire whose core is filled with metal powder such as a mixture of metal, alloy and/or oxide powders) is used with the wire typically at a positive electrical welding potential and with the parts electrically grounded. The welding arc creates a molten weld puddle which results in the welding together of the parts. Gas metal arc welding requires expensive welding equipment, the molten weld puddle tends to flow away from the joint area (depending on the joint position with respect to gravity) resulting in welds of inconsistent quality, and the process requires a long cycle time between welds. The need to precisely position the electrode at the joint and rotate around the tube circumference precisely so that the arc is drawn at the location to be welded is also difficult and time-consuming considering that thousands of tubes may be welded to a tubesheet to form a heat-exchanger. Any lack of precision in positioning the electrode at the joint results in leakers in the heat-exchanger.
Conventional methods for attaching parts together also include friction welding. To join a tube to a plate, the tube is rotated about its longitudinal axis, and the tube end and the plate are pressed together, wherein friction causes heating creating the weld. Friction welding requires expensive welding equipment, and the process requires a long cycle time between welds. Friction welding is not easily applicable to thin-walled tubes because they do not retain their shapes well under heat and pressure. Friction welding is not easily applicable to the manufacture of heat-exchangers, since the tubes are not commonly rotatable after assembly for welding. It is noted that laser and electron-beam welding for the above joints also need expensive equipment and there is the need for precisely positioning and rotating the heat source at the multitude of joints in a heat-exchanger.
What is needed is an improved welding apparatus for welding a heat exchanger tube to a tubesheet.